1. Field of the Invention
This invention relates to a method for manufacturing a lithium secondary battery that is capable of obtaining a lithium secondary battery having excellent cycle characteristics, the lithium secondary battery manufactured by this method, and a lithium secondary battery system.
2. Description of the Related Art
In the field of information-related devices or communication devices, a lithium secondary battery with high-energy density has been used practically and widely as the power source for such devices, due to the miniaturization of personal computers, video camera, cellular phones and the like. In the field of automobiles as well, use of a lithium secondary battery as the power source of an electrical vehicle has been considered, as the development of electric vehicles has been accelerated due to the environmental and resource problems.
Recently, various experiments have been conducted to improve the characteristics of a lithium secondary battery. For example, Japanese Patent Application Publication No. 2007-128723 (JP-A-2007-128723) discloses a battery in which the open circuit voltage when fully charged falls within a range of 4.25 V to 6.00 V and in which a lithium composite oxide such as LiCoO2 and/or lithium phosphate such as LiFePO4 is used as a positive-electrode active material and an electrolyte solution containing vinylene carbonate, supporting electrolyte (LiPF6) and Lithium Bis (Oxalato) Borate (LiBOB) is used. This technology aims to obtain a battery capable of improving the charge/discharge efficiency even when the upper limit of a charging voltage is set at 4.2 V or higher.
However, this technology is to mainly solve the problems of the lithium composite oxide such as LiCoO2. Specifically, in consideration of the fact that “in the lithium secondary battery that is operated at the maximum of 4.2 V, lithium cobaltate or other positive-electrode active material used in the positive electrode only utilizes approximately 60 percent of the theoretical capacity of the positive electrode” (the paragraph 0005 of JP-A-2007-128723), this technology aims to “improve the charge/discharge efficiency even when a charging voltage is set at 4.2 V or higher” (the paragraph 0008 of JP-A-2007-128723). In other words, the technology described in JP-A-2007428723 aims to enhance the Li discharged amount of the positive-electrode active material per unit weight by setting the upper limit voltage of charging higher than the normal level.
Japanese Patent Application Publication No. 2006-216378 (JP-A-2006-216378) discloses a nonaqueous electrolyte secondary battery in which a specific lithium composite oxide is used as a positive-electrode active material and in which an electrolyte containing LiPF6, LiBOB, and specific aromatic compound is used. This technology aims to prevent the deterioration of the cycle characteristics and battery swelling in high-temperature storage. Japanese Patent Application Publication No. 2005-285447 (JP-A-2005-285447) discloses a lithium ion secondary battery in which LiFePO4 is used as a positive-electrode active material and in which nonaqueous electrolyte solution containing γ-butyrolactone is used. This technology aims to provide a large lithium ion secondary battery having excellent safety and battery performance.
In JP-A-2007-128723 and JP-A-2005-285447, LiFePO4 is used as the positive-electrode active material. Generally, LiFePO4 has excellent thermal safety, large theoretical capacity of 170 mAh/g, and an insertion/elimination reaction of the lithium that progresses at a high voltage of approximately 3.4 V (vs. Li/Li+). Therefore, LiFePO4 is highly expected to be the positive-electrode active material for the next generation. However, the lithium secondary battery that uses LiFePO4 as a positive-electrode active material might not provide sufficient cycle characteristics.
For example, the following problems might possibly occur in a lithium secondary battery that uses LiFePO4 (positive-electrode active material), carbon material (negative-electrode active material) and LiPF6. Specifically, when charging/discharging is performed on the lithium secondary battery having such configuration, the LiPF6 contained in the nonaqueous electrolyte solution is decomposed, and PF5 or HF is generated, whereby the Fe component of LiFePO4 is eluted. Because the eluted Fe component breaks a solid electrolyte interface (SEI) film formed on the carbon material serving as the negative-electrode active material, the capacity is reduced to form the SEI film again. Consequently, deterioration of the cycle characteristics occurs.
On the other hand, because a lithium secondary battery that uses LiFePO4 (positive-electrode active material), carbon material (negative-electrode active material) and LiPF6 is normally charged/discharged at approximately 3.4 V, the upper limit voltage is normally set at approximately 3.6 V to 4.0 V at the time of charging. However, such voltage range does not contribute to the improvement of the cycle characteristics.